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Biofouling and control approaches in membrane bioreactors
Lijuan Denga,b, Wenshan Guob,d, Huu Hao Ngob,d*, Hongwei Zhanga,c, Jie Wanga,
Jianxin Lic,d, Siqing Xiad, Yun Wua
a School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, China b Centre for Technology in Water and Wastewater, School of Civil and Environmental Engineering, University of Technology Sydney, Broadway, NSW 2007, Australia c State Key Laboratory of Separation Membranes and Membrane Processes, Tianjin Polytechnic University, Tianjin 300387, China d Membrane Bioreactor Centre, College of Environmental Science and Engineering, Tongji University, State Key Lab. of Pollution Control and Resource Reuse, Shanghai, 200092, China
*Corresponding author, Email: h.ngo@uts.edu.au or ngohuuhao121@gmail.com; Tel: +61-2-
95142745
Abstract
Membrane fouling (especially biofouling) as a critical issue during membrane reactor
(MBR) operation has attracted much attention in recent years. Although previous
review papers have presented different aspects of MBR’s fouling when treating various
wastewaters, the information related to biofouling in MBRs has only simply or partially
reviewed. This work attempts to give a more comprehensive and elaborate explanation
of biofilm formation, biofouling factors and control approaches by addressing current
achievements. This also suggests to a better way in controlling biofouling by developing
new integrated MBR systems, novel flocculants and biomass carriers.
Keywords: Membrane bioreactor; Biofouling; Sludge properties; Operating conditions;
Biofouling control strategies
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1. Introduction
Membrane fouling, especially biofouling, is what primarily hinders the
application and development of membrane biroeactors (MBRs) (Meng et al., 2009).
Recently, some review papers have analysed membrane fouling in MBRs for
wastewater treatment. Meng et al. (2009) addressed fouling behaviour, fouling factors
and controlling strategies of MBRs for municipal and industrial wastewater treatment.
However, specific information regarding effects of sludge conditions (e.g. mixed liquor
suspended solids (MLSS), sludge viscosity) on membrane fouling and impacts of bound
EPS on sludge characteristics (e.g. flocculation ability, hydrophobicity) was not clearly
provided.
The fouling characterisation methods, results and fouling mitigation strategies
were updated and critically re-evaluated by Drews (2010). Nevertheless, some key
parameters affecting membrane fouling were not articulated in this review, such as EPS
and HRT. Le-Clech (2010) reported MBR applications, design and removal
performance, and current research trends. However, this review paper did not discuss
the HRT as one of the main parameters affecting soluble microbial products (SMP) and
membrane fouling. An overview on the current application of MBR technology for
industrial wastewater treatment was presented by Lin et al. (2012), who reviewed and
analysed more than 300 scientific publications on MBRs (including anaerobic MBR
(AnMBR)) for industrial wastewater treatment. However, since the objective of this
review was not to comprehensively present membrane fouling mechanisms and
characteristics, membrane fouling (including fouling factors and membrane fouling
propensity) was only briefly mentioned together with some unique and significant
findings concerning membrane fouling control measures. Although Mutamim et al.
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(2012) reviewed the performance of MBRs in treating high strength industrial
wastewater, which covered the operation, constraints and fouling mitigation strategies
in general views, the information was inadequate regarding the influence of membrane
properties, biomass and operational conditions on membrane fouling along with
membrane fouling mitigation. Similarly, a later review did not clearly address the
relationship between EPS/SMP and membrane fouling (Mutamim et al., 2013).
To date, several review papers have been published on biofouling in MBRs. Guo
et al. (2012) undertook a holistic review on different membrane fouling phenomena
during water and wastewater treatment, but only partially discussed biofouling in
membrane filtration processes. Wu and Fane (2012) reviewed factors (feed
characteristics, oxygen level, temperature, steady or unsteady-state operation) affecting
microbial behaviors, characterisation technologies of biofoulants and strategies for
controlling microbial relevant fouling (e.g. application of ozone, etc.) in MBRs when
treating industrial and municipal wastewater. Unfortunately, this review did not
document the effects of HRT on characteristics of microbial flocs and their metabolic
products, which affected membrane fouling. Nguyen et al. (2012a) highlighted the
underlying causes of membrane biofouling, emphasised the role of EPS in membrane
biofouling, and discussed recent developments in biofilm examination and biofouling
control in water and wastewater treatment. However, this review did not elaborate on
the impacts of operating conditions (i.e. HRT) and sludge characteristics (i.e. MLSS) on
membrane fouling. Later on, a comprehensive review conducted by Wang et al. (2013a)
focused on membrane fouling caused by EPS, including their secretion, transformation,
release, and adsorption in MBRs for municipal and industrial wastewater treatment.
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Nevertheless, new knowledge on sludge properties regarding membrane fouling, such
as MLSS and floc size was not articulated.
This review paper is dedicated to broader and more detailed analyses of the main
aspects of biofouling in MBRs based on current research achievements. A more
thorough discussion is carried out with respect to biofilm formation, the interactions
among the biomass characteristics, operating conditions, feedwater characteristics and
membrane properties, and their effects on membrane biofouling. Furthermore, some
helpful and reasonable suggestions are given in view of biofouling minimisation.
Finally, emerging fouling control strategies (e.g. membrane cleaning, biomass carriers,
flocculants, etc.) are presented along with future research trends in membrane
biofouling.
2. Biofouling and its effects on membrane systems
Of the six principle fouling mechanisms (pore blocking, cake formation,
concentration polarization, organic adsorption, inorganic precipitation and biofouling),
biofouling is the most complicated since it is associated with undesirable deposition,
growth and metabolism of bacterial cells or flocs on membrane surface and/or inside
membrane pores (Guo et al., 2012). Generally, six steps involved in biofilm formation
are shown in Fig. 1 (Bitton, 2005; Guo et al., 2012): (1) formation of a conditioning
film on the pristine surface after exposing the surface to the feed including organic
matters; (2) transport of microorganisms and planktonic cells to conditioned surface by
diffusion, convection, turbulent eddy, and chemotaxis; (3) adhesion of microorganisms
to conditioned surface as a result of the negatively free energy of adhesion, the balanced
force between Lifshitz-Van der Waals forces and repulsive or electrostatic forces from
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both microbial and substratum surfaces, or hydrophobic interactions; (4) continuous
attachment and adsorption of microorganisms on the surface due to the presence of EPS
which enables microorganisms to be adhered to the surface and bond with the biofilm;
(5) biofilm growth and accumulation; and (6) formation of a three-dimension biofilm.
Fig. 1.
The effects of biofouling on the performance of membrane systems are as follows:
(1) decreased membrane permeability resulting from bacterial attachment and
subsequent growth on the membrane surface; (2) precipitation of calcium carbonate at
an elevated pH; (3) accumulation of abiotic particles because of microorganism
adhesion or an enzymatic attack on the membrane or the glue lines; and (4) concentrated
polarization by biofilms on the membrane surface (Dreszer et al., 2013). During the
MBR process, biofouling results in poorer membrane permeability, more serious flux
decline, more frequent membrane replacement and higher operational and maintenance
costs.
3. Factors affecting biofouling in MBR
The biofouling factors discussed in this review paper include mixed liquor
properties, operating conditions, feed water characteristics and membrane properties.
3.1. Mixed liquor properties
3.1.1. Bound extracellular polymeric substances (EPS)
Bound EPS, which are mainly composed of polysaccharides (EPSC) and proteins
(EPSP), played a pivotal role in membrane biofouling development, especially cake
layer formation. In a MBR under long-term operation, biocake on the membrane surface
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mainly consisted of microbial cells and EPS, which evolved from an initially reversible
fouling inducing via the accumulation of easily removable organic substances to the
subsequent development of irreversible fouling; this in turn is due to consistently
increasing EPS levels (Domínguez et al., 2012).
Tian et al. (2011a) observed that more severe membrane fouling occurred during
filtration of bulking sludge, which was linked to the abundance of filamentous bacteria
and presence of more cell-bound EPS than that for the normal sludge. Excessive EPS
would deteriorate the structure of sludge flocs and reduce their bioflocculation ability.
Moreover, a higher EPSP/EPSC ratio in the bulking sludge increased floc hydrophobicity.
Thus, the sludge flocs having more EPS and higher EPSP/EPSC ratio could aggravate
membrane fouling by their deposition on the membrane surface. Liu et al. (2012)
discovered that higher levels of loosely bound EPS (LB-EPS) resulted in a poorer
flocculability and a lower settleability of activated sludge, and the formation of looser
sludge flocs, which increased cake layer resistance, total membrane resistance and TMP
development. It was also pointed out that a high concentration of LB-EPS obtained at a
high proportion of dead cells in activated sludge stimulated the generation of fine
particles and attachment of small flocs onto the membrane. This gave rise to the
formation of a dense cake layer on the membrane surface and serious membrane fouling.
However, tightly bound EPS (TB-EPS) reduced the incidence of membrane fouling. A
greater TB-EPS to LB-EPS ratio reflected a high flocculability of activated sludge, thus
encouraging the generation of large flocs, less membrane fouling and lower total
membrane resistance (Azami et al., 2011).
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3.1.2. Soluble microbial products (SMP) and biopolymer clusters (BPC)
When compared to other sludge characteristics (e.g. MLSS, particle size
distribution, bound EPS), SMP had the strongest relationship with membrane fouling
rate (dTMP/dt) (Zhang et al., 2015a). SMP (or soluble EPS) could be readily deposited
and adsorbed on and/or into the membrane owing to the permeation drag and higher
Brownian diffusion. In addition, the attachment of SMP to membrane surface could be
achieved by considerably high attractive interaction energy and through overcoming
high repulsive energy barrier. Thus, they would encourage the formation of a gel layer,
cause membrane pore blocking, and penetrate into the pores and spaces between
particles in the cake layer. In the gel and cake layers, SMP with macromolecular
character constituted the major soluble organic substances. When compared to the cake
layer, the gel layer possessed more SMP, which had unusually high specific filtration
resistance being almost 100 times higher than the cake layer. These effects led to
serious membrane fouling and decreased permeate flowrate (Hong et al., 2014; Wang et
al., 2012; Wu and Huang, 2009). Current studies revealed that the molecular weight
(MW) distribution of SMP impacted membrane fouling. Shen et al. (2010) and Shen et
al. (2012) pointed out that large-molecular-weight fraction (MW > 100 kDa) in
hydrophilic substances (HIS) of SMP, which mainly consisted of the polysaccharide-
like substances, engendered severe flux decline and pore plugging. Shen et al. (2010)
concluded that size exclusion played a key role in irreversible fouling of HIS.
Membrane fouling was also significantly influenced by SMP compositions
(especially proteins/polysaccharides (SMPP/SMPC) ratio). Yao et al. (2011) found that
higher SMPP/SMPC ratio reduced irreversible fouling and promoted the interaction of
SMPP and SMPC for cake layer formation on the membrane’s surface. However, Gao et
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al. (2013a) reported that when operating MBRs for treating industrial wastewaters from
a local thermomechanical pulping mill, higher SMPP/SMPC ratio negatively influenced
the cake layer structure (i.e. higher thickness, lower porosity, smaller floc size) and
increased the filtration resistance. Besides, Tian et al. (2011b) conducted a series of
stirred dead-end filtration tests to evaluate the fouling potential of biomass-associated
products (BAP) and utilization-associated products (UAP). The results implied that
membane interception induced almost complete reduction of proteins and partial
decrease of polysaccharides of UAP and BAP in the permeate. Membrane foulants were
mainly composed of proteins and polysaccharides. The modelling work demonstrated
that cake filtration and complete blockage were primary fouling mechanisms for BAP
and UAP filtration, respectively. This suggested that fouling potential for the UAP was
higher than that for BAP.
Fig. 2 shows a schematic illustration of the correlation between SMP and BPC in
MBRs. The interception of BPC by sludge layer, the aggregation and clustering of SMP
and small BPC as well as attachment of loose EPS within the sludge layer were
responsible for the formation and growth of BPC in sludge cake. BPC acted as glue and
thus prompted the biomass deposition on the membrane surface, which in turn
accelerated the formation of sticky and impermeable sludge cake. It resulted in serious
membrane fouling and high filtration resistance of the cake layer. On the other hand,
BPC could be detached from the membrane surface into the sludge suspension by the
turbulent shear resulting from aeration in MBR. Large BPC of activated sludge in MBR
will deteriorate due to the fluid turbulence, generating smaller SMP segments.
Subsequently, both SMP and BPC were in a transformable state regarding their size
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distribution and demontrated a balanced size distribution in mixed lquor in MBR (Sun
et al., 2008; Wang and Li, 2008).
Fig. 2.
3.1.3. Mixed liquor suspended solids (MLSS) and sludge viscosity
Some studies have explored the relationship between sludge viscosity and MLSS
concentration as well as their impacts on membrane biofouling. Table 1 displays the
effects of MLSS concentration on biofouling in MBR. Higher MLSS resulted in more
retention of sludge particles, colloids, macromolecular matter and microbial products in
the bioreactor. Higher sludge viscosity moderated the effects of air bubbling on
membrane fouling reduction, discouraged membrane fibre vibration, and allowed larger
air bubbles to move at low rise velocity. Moreover, higher viscosity also dampened the
back transport effect and increased the net force towards the membrane surface, leading
to the deposition of sludge flocs, biopolymers and smaller particles on the membrane’s
surface. Thus, activated sludge with higher MLSS levels and viscosity will compromise
membrane performance in terms of more rapid flux decline, lower membrane
filterability, higher TMP and greater membrane fouling resistance (Kornboonraksa and
Lee, 2009; Lay et al., 2010; Meng et al., 2007).
Table 1.
Current studies have pointed out that MLSS of 10 g/L was a critical point with
respect to membrane fouling. Wu and Huang (2009) suggested that high MLSS
concentration (> 10 g/L) stimulated a wide-range increase in sludge viscosity, which
showed a remarkable impact on membrane filterability (poor filterability). Lousada-
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Ferreira et al. (2010) collected activated sludge from full-scale MBRs and submitted the
sludge to cross-flow filtration by employing the same membrane and operating
conditions. MLSS concentrations were varied (3.6–18.3 g/L) by diluting the original
samples. MBR activated sludge at higher MLSS concentration (> 10 g/L) adsorbed
smaller particles (< 20 μm) and generated dilutions with worse filterability. Very
different results were obtained for the activated sludge samples at MLSS concentrations
below 10 g/L, which could not entrap particles.
3.1.4. Floc size
It has been indicated that the permeate flux (membrane permeability) was
positively correlated with sludge floc size (Kornboonraksa and Lee, 2009). From the
hydrodynamic viewpoint, large flocs could be dragged away from the membrane by
high shear-induced diffusion and inertial force as well as lower Brownian diffusion. The
deposition of larger and looser sludge flocs on the membrane encouraged the formation
of a more porous and permeable cake layer, thus reducing fouling resistance. On the
other hand, Brownian diffusion controlled the motions of small flocs at lower shear
stress (Dizge et al., 2011a; Pan et al., 2010). It is noted that membrane pore size should
be also taken into account when investigating the effects of sludge floc size on
membrane fouling. Shen et al. (2015) observed that when membrane pore size was 0.3
μm and most of flocs were larger than 1 μm, less obvious pore clogging occurred. The
thermodynamic analyses revealed that the specific energy barrier was slightly higher
when decreasing the sludge floc size, while smaller flocs promoted an increase in the
attractive specific interaction energy in contact. Hence, the whole effect favoured the
deposition of small flocs on the membrane and the subsequent formation of a less
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porous cake layer with small pore size, which in turn increased hydraulic cake
resistance and osmotic pressure-induced resistance as a result of more biopolymer
matters being retained in the pores of the cake layer.
3.2. Operating conditions
3.2.1. Sludge retention time (SRT)
It has been widely accepted that SRT is one of the most critical operating
parameters during MBR operation. Table 2 summarises the effects of SRT on
biofouling in MBR. When treating synthetic wastewater, SRT affected the microbial
community and further activated sludge characteristics (e.g. MLSS concentration, EPS
properties), which influenced the hydrodynamic conditions (the dynamic balance
between shear force and permeate drag along the membrane surface). It in turn
governed cake layer development and membrane fouling propensity. At infinite SRT,
membrane fouling (cake-fouling) was more severe due to the presence of smaller floc
size and more EPS than those at SRTs of 10 and 30 d when operating at a high flux of
30 L/m2·h (Wu et al., 2011a). Chen et al. (2012) suggested that SRT should be
maintained within a moderate range − 20 to 40 d. EPSC accumulated from SRT of 10 to
40 d caused by endogenous respiration, and increase in microbial death and intracellular
polymers. Meanwhile a prolonged SRT above 40 d led to a decrease in EPSC
concentration considering that inadequate nutrients supplied for microorganisms
prompted EPSC biodegradation. However, EPSC value at higher SRT (> 40 d) was still
higher than that at lower SRT (< 40 d). On the other hand, the reduction of supernatant
COD (SMP) was obtained at shorter SRT (< 40 d) because of their biodegradation.
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When SRT was extended, SMP accumulated due to the dissolution of intracellular
substances induced by the breakdown of dead cells and less SMP biodegradation.
Table 2.
When treating a pharmaceutical process wastewater at various SRTs (15 d, 30 d,
without sludge wasting (WSW)), both permeate volumes and steady state fluxes of the
submerged MBR (SMBR) increased at longer SRTs. This could be ascribed to the
decreased membrane fouling because of lower concentrations of EPS and SMP
(including protein and carbohydrate fractions). Total fouling resistance (RT) and cake
resistance (RC) decreased with increasing SRTs. RC was the major contributor for RT
under the WSW, while for 15 and 30 days of SRT, membrane pore resistance (RP)
increased when compared to the WSW (Kaya et al., 2013). Zhang et al. (2015a)
suggested that an anaerobic-oxic MBR at SRT of 30 d displayed better filtration
performance compared to SRTs of 10 and 90 d when treating campus sewage. Median
particle size and MLSS concentration in the MBRs followed the order of SRT = 90 d >
SRT = 30 d > SRT = 10 d. SMP concentrations were higher at SRTs of 10 and 90 d than
those at SRT of 30 d. SRT of 10 d gave rise to pore blocking and serious irreversible
fouling, while the formation of a cake layer and severe reversible fouling were attained
at SRT of 90 d.
Based on investigations above and taking into account the suggestion by Meng et
al. (2009), too long or short SRT was no advantage when attempting to control fouling.
The selection of SRT for MBR operation depended on membrane properties (e.g. pore
size), HRT and types of wastewater. MBR should be operated at SRT of 20–40 d when
treating synthetic wastewater, while for real wastewater treatment, it is recommended
that SRT be controlled at 20–30 d or WSW.
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3.2.2. Hydraulic retention time (HRT)
HRT is another key operating parameter having a pronounced impact on membrane
fouling (especially biofouling) during the MBR process, which can alter mixed liquor
properties, for instance EPS, SMP and sludge stability (Guo et al., 2012). Recent studies
compared MBR performance at various HRTs. When operating a lab-scale combined
system consisting of an anoxic tank and an aerobic MBR for digested wastewater
treatment, more severe membrane fouling and higher filtration resistance and fouling
rate were observed with decreasing HRT from 8 to 2.5 h. At shorter HRTs of 4 and 2.5
h, the membrane surface was covered by thick flocs that blocked most of membrane
pores (Gao et al., 2012). Babatouli et al. (2014) investigated the performance of an
MBR pilot plant treating industrial wastewater at a short HRT of 20 h. Their results
indicated that when compared to longer HRT of 24 h, shorter HRT of 19 h increased the
fouling rate and total concentrations of EPS and SMP as a consequence of more
substrate being made available for biomass at higher F/M ratio.
3.2.3. Dissolved oxygen (DO)
Air flow rate or DO is a principle operating parameter that determines the
hydrodynamic conditions, sludge characteristics (i.e. EPS, SMP), and membrane
fouling. When operating at low airflow rate or DO, cake layer would be formed by the
deposition of MLSS on the membrane, which could not be removed by the low shear
stress. Low DO also decreased aerobic activity of microorganisms, which reduced the
generation of EPS or increased anaerobic degradation of EPS, leading to the sludge’s
deflocculation. An appropriately high DO concentration could promote sludge
bioflocculation, and enhance sludge characteristics and membrane performance.
However, extremely high aeration intensity would break up sludge flocs, facilitate the
14
release of EPS, SMP and colloids, and deteriorate the filterability. It undermined
membrane performance because of the accumulation of colloids and solute within/on
the membrane (Braak et al., 2011; Faust et al., 2014). Gao et al. (2011) indicated that
more severe membrane fouling at low DO concentration (0.5 mg/L) was ascribed to
more EPS in membrane biocake than that at high DO concentation (4 mg/L).
At different DO levels, microbial community structure in mixed liquor and
biocake governed membrane fouling. The microbial community structure in biocake
displayed larger variations than that in mixed liquor. De Temmerman et al. (2015)
monitored a default aeration situation (reference) at DO of around 2 mg/L and an
elevated aeration situation (disturbance) in two identical MBRs, respectively. The later
one was accomplished by manually controlling aeration at DO concentration of around
4 mg/L for 18 h, after which the DO level immediately decreased to 2 mg/L (the
average setting, the reference situation). At high fine bubble aeration intensity, higher
concentrations of SMP, biopolymers, and submicron particles (10 1000 nm) were
obtained together with a shift in the particle size distribution (PSD) (3 300 m) towards
smaller sludge flocs. As a result, the system demonstrated higher total fouling due to the
formation of a less permeable cake layer and higher irreversible fouling mainly resulted
from pore blocking. Overall, an “optimal” air flow rate or DO concentration should be
determined considering loose/tight MBR configurations, and aeration time, position and
mode (e.g. cyclic aeration on/off).
3.2.4. Temperature
It demonstrated that the decreased temperature (7, 15, and 25 °C) prompted the
accumulation of polysaccharides in the soluble fraction of mixed liquor and a shift in
floc size of the soluble fraction towards smaller particles. This in turn exacerbated the
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occurrences of pore blocking/pore narrowing and membrane fouling (van der Brink et
al., 2011). Gao et al. (2013b) found that in both mixed liquor and biocake, the decrease
in temperature from 30 to 20 °C hastened the reduction of EPS and SMP (mainly EPSP
and SMPP). However, an additional decline in temperature to 10 °C resulted in an
obvious rise of EPS, SMP, EPSP and SMPP, thus aggravating membrane fouling. Both
EPSC in biocake and SMPC/SMPP in mixed liquor were significantly correlated with
temperature and filtration resistance. Another study implied that during the operating
period (8.7−19.7 ºC), the decline in temperature encouraged the increase of EPS and
SMP through promoting their formation and lowering hydrolysis ability of
microorganisms, which triggered serious membrane fouling.
3.3. Feedwater characteristics
3.3.1. Organic loading rate (OLR) or food/microorganism (F/M) ratio
The OLR or F/M ratio could change microbial behaviours, microbial properties
and biomass characteristics in mixed liquor (e.g. MLSS, particle size, viscosity, floc
structure, EPS and SMP) and membrane biofilm (e.g. bacteria community), thus
influencing membrane biofouling. As compared to the low loading (0.33 g
COD/(gVSS·d)), earlier and faster membrane fouling (irreversible fouling) was attained
under the high loading (0.52 g COD/(gVSS·d)) because of higher EPS contents,
substantially thick biofilm, and the dominance of two bacteria Betaproteobacteria and
Bacteroidetes in the biofilm during the long-term operation (Xia et al., 2010). Wu et al.
(2011b) reported that at various organic loadings, variations in F/M ratios and DO levels
led to changes in bacteria communities and EPS generation during the initial operational
period. At the steady state, a slightly lower membrane fouling in the low-organic-
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loading MBR (0.57 kg COD/m3·d) but remarkably higher fouling and specific fouling
reisistance in the high-organic-loading MBR (2.28 kg COD/m3·d) were achieved. This
was comparable to the medium-organic-loading MBR (1.14 kg COD/m3·d) at a high
flux of 30 L/m2·h. Higher organic loading increased EPS levels and facilitated the
formation of cake fouling on the membrane surface, whereas the increased organic
loading within a moderate range (0.57−1.14 kg COD/m3·d) helped to control fouling.
On the other hand, a short-term starvation (low F/M ratio of 0.05/d) stimulated activated
sludge to consume UAPs for exogenous metabolisms and retaining their activity, which
was conducive to fouling reduction as a result of SMP decline. However, a long-term
starvation would prompt the release of large amounts of high-molecular-weight BAP
(humic-acid substances) from SMP, which clogged the overall membrane surface and
subsequently led to serious fouling (Wu and Lee, 2011).
3.3.2. Carbon to nitrogen or carbon to phosphorus ratio (C/N or C/P)
Current studies focused on nutrient parameters, such as C/N, C/P ratios, which
had an effect on bacteria communities and sludge characteristics, thereby influencing
the potential for membrane biofouling. Compared to higher C/N ratio of 10:1 (lower
NH4+ concentrations), higher concentrations of NH4
+-N in the reactor at lower C/N ratio
of 5:1, acting as a monovalent cation, replaced the polyvalent cations in EPS, which
prompted the release of EPS components to form new SMP. Furthermore, SMP at lower
C/N ratio possessed more SMPC (hydrophilic neutrals), higher SMPC/SMPP ratio and
smaller MW distribution. It gave rise to higher irreversible fouling resistance, which
was well correlated with fouling profiles, i.e. higher TMP increment rate (Feng et al.,
2012). Wu et al. (2012) indicated that compared to the control MBR (COD/N/P =
17
100:8.7:3.5), an increased nitrogen or phosphorus loading in MBRs (low C/N-MBR
(COD/N/P = 100:17.4:3.5) or C/P-MBR (COD/N/P = 100:8.7:7.0)) triggered apparent
shifts of bacterial communities in a similar way. When the low C/N- or C/P-MBR
almost reached a steady state, biomass concentrations and EPS levels were comparable
to those in the control MBR. In addition, dominant bacteria community were similar in
both MBRs. Biomass samples from the low C/N-MBR and low C/P-MBR demonstrated
larger flocs and lower membrane fouling rate as compared to the control MBR.
3.3.3. Salinity and cations
Previous studies indicated that high salt concentration broke the multivalent cation
bridging among EPS within the sludge matrix, or reduced populations of protozoans,
resulting in sludge deflocculation, and poor sludge quality, and compromised
mechanical integrity and structure of sludge flocs. It reduced sludge filterability, whilst
exacerbating membrane biofouling (De Temmerman et al., 2014; Jang et al., 2013).
Jang et al. (2013) investigated MBR performance at various salinity conditions (NaCl,
5−20 g/L) and found that the elevated salinity changed microbial composition, which
influenced MBR performance. High salt concentrations increased fouling resistance due
to the increased pore blocking and TMP increment rate, and this situation was mainly
attributed to biomass properties (i.e. EPS, floc size and zeta potential). De Temmerman
et al. (2014) asserted that the salt shock in the MBR (NaCl addition in the influent as a
step up from 0.1 (background concentration) to 2 g/L lasting for 5 h) postponed the
increase in submicron particles, SMP levels (proteins and polysaccharides), and
irreversible fouling rate. There was no time delay in the shift in the supramicron particle
size distribution towards smaller floc sizes and an increase in total fouling rate.
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In feed water, divalent cations (i.e. calcium, magnesium) improved aggregation
and stabilization of matrix of biopolymers, microbes and sludge flocs through bridging
negatively functional groups within EPS, which alleivated membrane biofouling.
Normally, a relatively lower monovalent to divalent cation ratio (< 2:1) but higher
molar ratio of Mg:Ca (> 1:1) favours floc agglomeration (Arabi and Nakhla, 2009).
3.4. Membrane characteristics
Membrane characteristics (e.g. materials, morphology, pore size, zeta potential,
hydrophobicity/hydrophibicity affinity) are the principal factors affecting membrane
biofouling. A current study from Nittami et al. (2014) revealed that when using the
municipal wastewater treatment plant (WWTP) sludge which showed higher
hydrophobicity, a dense cake layer formation on the membrane and higher TMP
increase rate were achieved in the MBR with an immersed hydrophilic membrane than
those for the MBR with an immersed hydrophobic membrane. However, for the lab-
reactor sludge with lower hydrophobicity, the application of hydrophobic membrane
triggered faster TMP increment. Thus, the effects of membrane surface hydrophilicity
on membrane fouling were associated with the hydrophobicity of suspend sludge. It
should be also noted that surface hydrophilicity/hydrophobicity of membranes was an
essential factor for the interaction between membrane and foulants when the two
contacting surfaces are infinite planar. However, when foulants are spherical particles as
indicated by Zhang et al. (2015b), a different conclusion is reached. The interfacial
interaction between sludge foulants and membranes was more remarkably affected by
membrane surface zeta potential and roughness than membrane hydrophilicity/
hydrophobicity.
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Membrane fouling control is possible for membranes with relatively higher zeta
potential and certain roughness, which increased the strength of the electrostatic double
layer interaction and the energy barrier, and consistently maintained the total repulsive
interaction, respectively. Dizge et al. (2011b) investigated the type and pore size of
membranes on membrane fouling employing cellulose acetate (CA), polyethersulfone
(PES), mixed ester (ME), polycarbonate (PC) membranes with three different pore sizes
(0.40 0.45, 0.22, 0.10 m) in cross-flow microfiltration experiments. When the pore
sizes of the membranes decreased (e.g. PES and ME), RP decreased as larger flocs were
deposited on the surface of these membranes to form a filter cake or they clogged
membrane pore entrances instead of passing through the pores. Concentration
polarisation resistance was higher in the case of PES and ME membranes with smaller
pore sizes, which was resulted from adsorption of soluble fraction of microbial products
(carbohydrate and protein) onto the membrane surface. When compared to the clean
membranes, the surface of fouled CA, PES and ME membranes indicated less
roughness (smoother) because their surface cavities were covered by the microbial flocs.
Through evaluating effects of membrane pore size, surface porosity, pore
morphology, and hydrophobicity on membrane fouling, it was suggested that less
fouling could be achieved for the membrane with higher hydrophilic property,
asymmetric structure, interconnected pore morphology, relatively large pore size and
high surface porosity (van der Marel et al., 2010).
4. Biofouling control strategies
4.1. Membrane cleaning
20
Currently, membrane cleaning is classified into four types, namely, physical,
chemical, physico-chemical and biological/biochemical in view of fouling removal
mechanisms or the used cleaning agents. Physical and/or chemical cleaning methods
could damage membrane properties, encourage the release of chemical reagents and the
formation of cleaning by-products, which are detrimental to membranes, the
environment, microbial community and cell activity. To overcome the issues of above
cleaning approaches, biological/biochemical cleaning (e.g. mild cleaning alternatives,
such as enzymatic cleaning, energy uncoupling and quorum quenching) has also been
widely employed for the removal of membrane foulants. These methods are more
efficient in decreasing biofilm formation with less adverse effects on microbial activity,
microbial community, and membrane properties (Wang et al., 2014).
4.2. Addition of media
The direct addition of biomass carriers into MBRs could improve characteristics
of mixed liquor and cake layer, thus alleviating membrane fouling. Jin et al. (2013)
elucidated that ceramic MBR with suspended carriers (AnoxKaldnes, K1 carriers)
having relatively low density and large size contained larger biomass flocs, less SMP
and less EPS than that without the carriers. Furthermore the biomass carriers can reduce
the cake layer on membrane surface but only slightly affect pore blocking.
The fresh powdered activated carbon (PAC) in a submerged MBR at relatively
high concentrations eliminated EPS, SMP, fine colloids, polysaccharides, total organic
carbon (TOC) and planktonic cells in the supernatant of mixed liquor by synergistic
effects of adsorption, decomposition, and biodegradation. Furthermore, PAC promoted
the production of large, strong and dense sludge flocs, which limited deposition of
21
particles on the membranes and release of foulants, and retarded gel layer formation.
The fresh PAC could also accelerate the combined effects of collision and adsorption on
the membrane surface. Consequently, adding PAC led to less membrane pore blocking,
formation of a more porous, less compact and more stable cake layer on membrane
surface, lower fouling rate, longer lifespan of membrane, and less energy consumption.
Another advantage took the form of lower operating costs for cleaning the membrane or
replacing it (Jamal Khan et al., 2012; Ng et al., 2013; Skouteris et al., 2015).
When compared to the conventional MBR (CMBR), sponge in the SSMBR
reduced TMP development rate, and sustained good microbial activity and stable sludge
volume index (SVI) value (Nguyen et al., 2012b). Deng et al. (2014) elucidated that
sponge reduced EPS and SMP, lowered biomass growth, sludge viscosity and growth of
filamentous bacteria, increased zeta potential and relative hydrophobicity (RH), and
enlarged sludge floc size, thus reducing RC, RP and membrane fouling propensity in the
SSMBR.
Other kinds of additives such as diatomite and clinoptilolite have successfully
been employed in MBRs for membrane fouling reduction. They enlarged sludge floc
size, increased MLSS, reduced foulants (e.g. dissolved organic matter (DOM)) in mixed
liquor, and enhanced sludge settleability through the combined effects of adsorption and
co-precipitation on fine colloids and DOM (Rezaei and Mehrnia, 2014; Yang et al.,
2010).
4.3. Addition of flocculants
Adding different kinds of flocculants in MBRs could alter the properties of
activated sludge and cake layer and lead to enhanced membrane performance. The
22
effects of inorganic flocculants (FeCl3 and polyaluminium chloride (PACl)) and natural
organic flocculant (chitosan) on the performance of a submerged MBR were evaluated
by Guo et al. (2010) in short-term experiments. The results suggested that MBR with
addition of natural organic flocculants displayed relatively constant sludge volume
indexes and higher specific oxygen uptake rates. Lower SMP levels and membrane
fouling rates were achieved when applying inorganic flocculants in MBR. Zhang et al.
(2014) pointed out that at high salt shock, application of organic flocculant (especially
MPE50) could increase floc size, relative hydrophobicity, zeta potential and bound EPS
(especially EPSP), and reduce SMP by entrapping onto flocs, thus effectively mitigating
membrane fouling.
Nguyen et al. (2010) showed the effectiveness of a new combined inorganic-
organic flocculant (CIOF) of FeCl3 and MPE50 for membrane fouling control in an
aerated submerged MBR. Moreover, a stable SVI and low TMP development rate were
also realised by CIOF addition. Ji et al. (2014) presented that a polyacrylamide-starch
composite flocculant (PAM-MGMS) in submerged MBRs not only demonstrated a
long-lasting effect on SMP removal, but also extended the duration of floc size
reduction caused by the irreversible deterioration of sludge flocs under continuous shear
stress as well as the degradation of the modified starches. Therefore, PAM-MGMS
could be suggesting as an additive for fouling control.
As the aforementioned flocculants exerted detrimental impacts on the
environment and promoted the generation of “secondary pollutants” during wastewater
reclamation and reuse, a new green bioflocculant (GBF) as a safe biodegradable natural
flocculant was developed by modifying a natural starch-based cationic flocculant
(HYDRA Ltd., Hungary) (Ngo and Guo, 2009). A patented GBF was then developed at
23
University of Technology, Sydney (UTS), namely Gemfloc®. The results of a recent
study (Deng et al., 2015) suggested that better sludge characteristics were given by the
MBR with Gemfloc® addition (MBR-G), as demonstrated by less SMP, larger floc size,
higher zeta potential and greater RH, lower growth of suspended biomass and lower
sludge viscosity. Moreover, Gemfloc® reduced the contents of EPSP, SMP and BPC in
the membrane cake layer. Therefore the MBR-G realised lower membrane fouling
potential, RC and RP.
4.4. Other methods
As aerobic granule sludge featured an apparently large floc size with regular and
compact structure, desirable settleability, high biomass, various bacteria community and
high metabolic activity, Wang et al. (2013b) employed the aerobic granular sludge in a
MBR for membrane fouling control at a high flux of 20 L/m2·h. The results implied that
in comparison to the MBRs with flocculent sludge, bulking sludge and smaller aerobic
granular sludge (average diameter of 434 m), the good aerobic granular sludge (larger
average diameter of 903 m) in the MBR significantly extended the filtration duration
and reduced membrane fouling. Membrane fouling in the aerobic granular sludge was
mainly attributed to RP, while RC dominated the filtration resistance for flocculent
sludge or bulk sludge due to the deposition of small particles on membrane surface,
which prompted serious cake fouling. Recent studies also focused on the development
of electro-MBRs (e-MBRs) for fouling minimisation. A recent study by Zhang et al.
(2015c) revealed that a low-voltage electro-MBR (e-MBR) using stainless steel mesh as
the anode, namely Fe-MBR, lowered TMP, decreased irreversible fouling and enhanced
pollutant removals as compared to another e-MBR with titanium anodes (Ti-MBR) and
24
one MBR without an electrical field. The electrical field exerted the electrical field force
on negatively charged foulants, which moderated their tight attachment on the
membrane surface and helped remove the fouling layer. Additionally the release of iron
resulted in the reduction of TOC and polysaccharides of SMPs. Therefore less
irreversible fouling and lower overall fouling rate were realised in the Fe-MBR.
5. Challenges and perspectives
Combining MBR with other technologies has been considered as a promising
research trend. Although the current literature and discoveries have focused on this
approach, a comprehensive biofouling framework needs to be developed. Thus, more
efforts should be made to thoroughly analyse different MBR systems’ impact on
biofouling (including the above-mentioned integrated MBRs). The advanced biofouling
mitigation methods could be created based on the aforementioned studies. More
specifically, characterisation of bacterial communities and MW fractions of foulants
(EPS, SMP, and BPC) in both mixed liquor and cake layer, and biocake architecture
should be done when operating integrated MBRs. It would be also interesting to
conduct the meso-scale analyses and studies on micro-interfacial process in these
integrated MBRs to have more in-depth understanding of membrane fouling behaviour.
Apart from that, the effects of salt and temperature shocks on the performance of these
integrated MBRs must be carried out in detail to evaluate their feasibility for biofouling
control under stressful conditions. Future studies could be devoted to combining the
SSMBR or the SSMBR-G with biological/biochemical cleaning methods into one
bioreactor, i.e. immobilising the quorum-quenching bacteria into a sponge. Moreover,
25
the development of novel or modified flocculants and biomass carriers (e.g. using
sponge) is to be encouraged in future studies.
6. Conclusions
This review presented the current achievements in key biofouling factors and
recent developments in biofouling control strategies. BPC, EPS and SMP
concentrations and their compositions largely influenced sludge properties and
membrane fouling behaviour. Smaller floc size deteriorated membrane permeability.
The optimum MLSS level (< 10 g/L), SRT (20−40 d) and temperature (15−30 ºC)
should be maintained to minimize membrane fouling. The impacts of membrane
properties were partially determined by sludge characteristics and feedwater
characteristics. The integration of MBR with other approaches could effectively
enhance MBR performance. Future work should elucidate fouling behaviour in
integrated MBRs more clearly.
Authors’ note
Two Tables S1 and S2 are included in the supplementary material.
Acknowledgement
The authors are grateful for the research collaboration between University of
Technology Sydney with Tianjin Polytechnic University and Tongji University on
MBRC. The supports from the Opening Research Fund of State Key Laboratory of
Hollow Fiber Membrane Materials and Processes, 2016 (M3-201502) and the joint
University of Technology Sydney–China Scholarship Council (UTS–CSC) Doctor of
Philosophy (Ph.D.) Scholarship are appreciated.
26
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Fig. 1. Schematic of biofilm formation on a surface (modified from Bitton, 2005)
Fig. 2. Schematic illustration of correlation between SMP and BPC in MBR
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Fig. 1.
39
Fig. 2.
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Table 1. Effects of MLSS concentration on biofouling
Type of wastewater
MLSS concentration Effects on biofouling References
Synthetic wastewater
3.0-25.0 g/L
• At low MLSS levels (< 5.0 g/L), less membrane fouling was observed at the elevated MLSS concentration, while fouling rate was almost zero when MLSS concentration increased to 7.9 g/L;
• When increasing MLSS concentration (> 10 g/L) by two times, fouling rates increased by almost three times. Nine-fold increase in fouling rate was obtained over a three-fold increase in MLSS concentration (> 15 g/L).
Li et al., 2012;
Lee and Kim, 2013
Real wastewater
2.3-20.0 g/L
• A slight change of permeation flux was attained when increasing biomass concentration within low MLSS range (< 4.1 g/L);
• At higher MLSS concentration (4.1−6.75 g/L), increasing MLSS notably decreased permeate flux;
• The increase in MLSS from 5 to 20 g/L reduced the critical flux by 3 times.
Bani-Melhem et al., 2015; Damayanti et al., 2011
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Table 2. Effects of SRT on biofouling
SRT Effects on sludge characteristics and biofoulinga References
3 50 d (HRT = 6 26 h)
• Shorter SRT facilitated the accumulation of carbohydrates and proteins of SMP and EPS in the submerged MBR. In addition, it also increased the amounts of small molecules (MW < 1 kDa) of SMP and reduced macromolecules (MW > 30 kDa) of EPS. More undefined microbial by-product-like substances and distinct O-H bonds in hydroxyl functional groups were found with decreasing SRT;
• At lower SRT, MBR sludge featured poorer filterability and settleability, higher RH, and more filamentous bacteria. It led to more loose floc structure, poorer floc stability, poorer bioflocculation of activated sludge and generation of larger amounts of smaller aggregates;
• More severe membrane fouling, less porous fouling layer and a relative higher specific resistance were attained at lower SRT (i.e. 10 and 15 d). Besides, more distinct fouling layer on the membrane was formed at lower SRT (23 d), containing significantly higher concentrations of deposited EPS (40-fold higher for protein and 5-fold higher for carbohydrate) than those at higher SRT (40 d).
Al-Halbouni et al., 2008; Duan et al., 2014; Tian and Su, 2012; Van den Broeck et al., 2012
a EPS, extracellular polymeric substances; MW, molecular weight; RH, relative hydrophobicity; SMP, soluble microbial products
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Graphic abstract
43
Highlights
• Biofouling has become a major issue during MBR operation.
• Various MBR biofouling factors and control strategies are reviewed.
• Sludge properties play a critical role in membrane biofouling potential.
• Adding sponge or bioflocculant into MBR achieves desirable fouling reduction.
• Development of integrated MBRs with novel flocculants and media is needed.
Recommended